Many parts for automobile such as edge of a piston top ring part, a brake disk rotor part and a bulb lifter part require resistance to abrasion at a sliding zone and thus have been made of an aluminum composite material in which matrix metal is combined with reinforcement material having resistance to abrasion.
As a method for preparing the aluminum composite material, there is well known a so called melt-stirring method, in which a melt of aluminum is mixed with 5 to 30 wt.% of the reinforcement powders such as SiC while being stirred and then is cast into a product or element. Therefore, throughout the parts of the resulting product, besides the sliding zone necessary to be reinforced, the reinforcement material is distributed. Additionally a pouring gate and a feeder head used for casting also have the reinforcement material. The thus total amount of the reinforcement material to be used becomes larger and costly.
There has been requested to provide a method for making a partially reinforced composite product in order to decrease the production cost and there was proposed a so-called high pressure casting method, as shown in Japanese Patent Kokai Hei No. 3-151158, in which a mixture of SiC whiskers (short fiber crystalline having a diameter of micron order) and aluminum based metal powders is sintered into a composite preform having a determined shape, which is further provided at the surface with a thin layer of noble metal based material. The composite preform is set at a determined position in a mold, into which a melt of aluminum based metal is poured under a high pressure (for example 1000 kg/cm.sup.2). Thereby, the resulting product comes to be reinforced partially due to the composite matrix covered with the melt.
However, large amount of SiC whiskers must be still used as the reinforced material because SiC whiskers are fibers. Therefore, it results in high production cost and also results in high damage against the other partner parts. Further, SiC whiskers are difficult to be adjusted with respect to mixture ratio between the aluminum matrix and the reinforcement (SiC) and thus result in high volume ratio (Vf) of SiC whiskers in the composite matrix, even though a composite material having a low volume ratio is required in practical use.
Therefore, it is an object of the present invention to provide a method for preparing a light metal based composite material having a low volume ratio of the reinforcement material.
According to a first aspect of the present invention, there can be provided a method for preparing a light metal or a light metal alloy based composite product, which comprises steps:
(a) preparing an aqueous slurry comprising water, reinforcement powders and a binder;
(b) preparing a preform mainly composed of said reinforcement powders and binder from said slurry;
(c) sintering said preform;
(d) impregnating the sintered preform in a mold with a melt of aluminum or a light metal alloy to give aluminum or a light metal alloy based composite product.
The composite material products thus prepared result in a partially reinforced light metal based composite product with an optional volume ratio of the reinforcement material, especially a low volume ratio which is actually required.
The method according to the present invention can be applied to a matrix metal selected from the group consisting of aluminum, Al alloys and Mg alloys such as AZ91D.
The aqueous slurry can be prepared from water, the reinforcement powders and the binder. The reinforcement powder is used for reinforcing the matrix metal in terms of resistance to abrasion or corrosion and also improvement in mechanical strength, which is selected from 1) ceramic powders and 2) metal powders for making an intermetallic compound with said light metal or light metal alloy (matrix metal). The ceramic powder may be selected from the group consisting of SiC, SiN, TiO.sub.2 and Al.sub.2 O.sub.3. On the other hand, the metal powder may be selected from the group consisting of Ni, Cu, Fe and Ti. In case the ceramic powder is SiC, the average size of SiC powders is preferably within 10 to 20 .mu.m, especially about 15 .mu.m, because less than 10 .mu.m lowers the resistance to abrasion of the resulting composite product while more than 20 .mu.m deteriorates the processability. In case the metal powder is Ni, the average size of Ni powders is preferably within 20 to 40 .mu.m, especially 30 .mu.m, because less than 20 .mu.m does not improve resistance to abrasion of the composite product while more than 40 .mu.m deteriorates the processability.
The binder is used to bind the reinforcement powders in the preform and among many kinds of the binders, alumina sol is preferred considering bonding strength between the reinforcement powders. The alumina sol is an alumina hydrate boehmite which has colloids of about 5 to 200 m.mu. and contains a stabilizer such as Cl--, CH.sup.3 COO--, NO.sub.3 -- and so on. Kinds of alumina sol-100, 200 and 520 are available. The mixture ratio between the reinforcement powders and the binder should be determined considering the form retention of the preform. In case no aluminum or aluminum alloy powders are added with slurry, there should be added 0.5 to 5 wt.% of alumina sol based on the weight of the reinforcement material. On the other hand, in case aluminum or aluminum alloy powders are added in the slurry, there should be added 0.5 to 3 wt.% of alumina sol based on the total weight of the reinforcement material and the aluminum or aluminum alloy powders.
The slurry may further comprise a volatile material for making spaces, to be filled with the matrix metal, in the preform due to volatilization during the sintering step. The volatile material may be selected from graphite or organic material such as phenol resins. The volatile material has average size of 35 to 55 .mu.m and the content may be within a range from 0 to 35 volume %, based on the volume of the preform. The resulting volume ratio of the preform is preferably within a range from 5 to 30 %. In case of the preform should have 20 volume ratio, the slurry is preferably prepared by mixing 20 vol.% of the reinforcement material and 20 vol.% of the volatile material. Further, in case aluminum, Al alloy or Mg alloy is a matrix of said composite product, the slurry may further comprise aluminum powders, Al alloy powders and/or Mg alloy powders. In case of Mg alloy powders, Mg particles of more than 200 .mu.m should be used. Therefore, the preferred slurry may comprise water, the reinforcement powders, the binder, aluminum powders and/or light metal alloy powders and the volatile material for making spaces, such as graphite.
In preparing the preform, since the slurry contains much water, it is preferred that, the preforming step (b) can be carried out by (1) dehydrating the slurry in a preform mold to prepare a preform mainly composed of said reinforcement powders and binder; and (2) compressing the dehydrated preform.
In the sintering step, while the preform nay be sintered at a temperature lower than the melting point of the matrix metal, a sintering temperature higher than the melting point may be selected depending on the desired strength of the preform. However, since alumina sol can be crystallized to .gamma.-Al.sub.2 O.sub.3 at a temperature higher than 485.degree. C., preventing alumina sol from reacting with Mg in the matrix, it should be carried out at a temperature higher than 500.degree. C. Otherwise, the alumina sol reacts with Mg in the matrix, resulting in insufficient extraction of Mg.sub.2 Si during the heat treatment for improving the mechanical strength of the composite product. In case metal powders, such as Ni powders, are used in the slurry forming the intermetallic compound, the sintering temperature may be higher than 530.degree. C. in order to be able to form the metal intermetallic compound with the matrix metal. In case the matrix metal is other than the reinforcement material to make the slurry composition, the sintering temperature may be preferably higher than its melting point (for example, aluminum based alloys: about 570.degree. C.; their powder: 530-540.degree. C.) because the sintering temperature causes melting of the matrix metal which makes the reinforcement powders to be combined together strongly in a network form with the aid of the alumina sol. Therefore, the preform, especially composed of the aluminum based alloy may be sintered at a temperature higher than 580.degree. C., but should not be done above 900.degree. C. because such a high temperature may fail to keep the form retention of the preform.
In the present invention, the resulting composite product is preferred to be subjected to T6 heat treatment since the product contains sufficient Mg due to sufficient transfer from the alumina sol to .gamma.-alumina during the sintering step, so that the T6 heat treatment makes Mg component to combine Si component to form Mg.sub.2 Si which improves the mechanical strength of the resulting composite product.